1. Field of the Invention
This invention relates to pneumatic tires, and more particularly to a pneumatic tire simultaneously satisfying good wet-skid resistance, rolling resistance and wear resistance.
2. Related Art Statement
Recently, the study for reducing the rolling resistance of the tire become important for saving the fuel consumption in automobiles in accordance with social demand for saving energy and resources. In general, it is known that as the rolling resistance of the tire is reduced, the fuel consumption in the automobile is mitigated to provide a low fuel consumption tire. In order to reduce the rolling resistance of the tire, generally a material having a small hysteresis loss as a tread rubber for the tire can be used. On the other hand, the use of a rubber material having a large friction resistance (wet-skid resistance) on wet road surface is strongly desired from the viewpoint of the demand for the running stability. However, the low rolling resistance and the high friction resistance on wet road surface are conflicting with each other, so that it is very difficult to simultaneously satisfy these properties.
Notably, the viscoelastic properties in the rubber composition are theoretically related to the wet-skid resistance and rolling resistance of the tire. That is, it is indicated to effectively reduce the fuel consumption by decreasing the hysteresis loss of the tread rubber for reducing the rolling resistance during the running of the tire, i.e. by viscoelastically decreasing the loss factor (tan .delta.) at a temperature of 50.degree.-70.degree. C. used in the running of the tire. On the other hand, it is known that the wet-skid resistance is well interrelated to the loss factor (tan .delta.) at about 0.degree. C. at a frequency of 10-20 Hz. Therefore, it is required to make the loss factor at about 0.degree. C. large in order to improve the gripping performance of the tire.
As a method of decreasing the hysteresis loss, generally a material having a low glass transition temperature can be used such as high-cis polybutadiene rubber and the like, or a material having a high rebound resilience such as natural rubber and the like. However, the use of these rubbers lowers the wet-skid resistance, so that it is considerably difficult to simultaneously establish the running stability and the low rolling resistance.
However, many techniques utilizing anion polymerization have been proposed in order to solve the aforementioned problems. For example, Japanese Patent laid open No. 55-12133 and No. 56-127650 disclose that high vinyl content polybutadiene rubber is effective for solving the problems, and Japanese Patent laid open No. 57-55204 and No. 57-73030 disclose that high vinyl content styrene-butadiene copolymer rubber is effective for solving the problems. Furthermore, Japanese Patent laid open No. 59-117514, No. 61-103902, No. 61-14214 and No. 61-141741 disclose that the heat generation is reduced by using a modified polymer obtained by introducing a functional group such as benzophenone, isocyanate or the like into a molecular chain of the polymer. Even in these techniques, the low value recently required for the rolling resistance is not yet sufficient.